The present invention generally relates to projection exposure methods and optical masks, and more particularly to a projection exposure method necessary for the fine photolithography technique which is used in the process of producing semiconductor integrated circuit devices, and an optical mask which is used in such a projection exposure.
Because the operation speed and integration density of semiconductor integrated circuit devices have increased in recent years, there are demands to establish photolithography techniques which can form even finer patterns.
Various phase shift methods have been proposed as photolithography techniques capable of forming fine patterns. The phase shift method uses the phase difference of lights which are transmitted through parts of an optical mask having mutually different thicknesses. However, it is still impossible to completely satisfy the above described demands using the phase shift method.
According to the conventional phase shift method, it is essential to use a phase shift mask which is formed with a high accuracy. However, there are problems in that it is extremely difficult to produce, inspect and correct the phase shift mask which is formed with the high accuracy.